Quantum dot sensitized wide bandgap semiconductor photovoltaic devices &amp; methods of fabricating same

ABSTRACT

A quantum dot (QD) sensitized wide bandgap (WBG) semiconductor heterojunction photovoltaic (PV) device comprises an electron conductive layer; an active photovoltaic (PV) layer adjacent the electron conductive layer; a hole conductive layer adjacent the active PV layer; and an electrode layer adjacent the hole conductive layer. The active PV layer comprises a wide bandgap (WBG) semiconductor material with E g ≧2.0 eV, in the form of a 2-dimensional matrix defining at least two open spaces, and a narrower bandgap semiconductor material with E g &lt;2.0 eV, in the form of quantum dots (QD&#39;s) filling each open space defined by the matrix of WBG semiconductor material and establishing a heterojunction therewith. The active PV layer is preferably fabricated by a co-sputter deposition process, and the QD&#39;s constitute from about 40 to about 90 vol. % of the active PV layer.

FIELD OF THE INVENTION

The present invention relates to improved quantum dot sensitized widebandgap semiconductor photovoltaic devices and methods for fabricatingsame. The invention has particular utility in the manufacture and use ofphotovoltaic solar cells for direct conversion of radiant to electricalenergy.

BACKGROUND OF THE INVENTION

A recently developed class of photovoltaic (“PV”) cells and devices,sometimes referred to as “Graetzel Cells” (see, e.g., U.S. Pat. Nos.4,684,537; 5,350,644; 5,463,057; 5,525,440; and 6,278,056 B1), is basedupon dye-sensitization of a wide bandgap (“WBG”) semiconductor (i.e.,E_(g)≧2 eV, preferably ≧3 eV), such as TiO₂, for efficient injection ofelectrons into the conduction band upon photo-excitation of the dyemolecules, followed by charge separation and photovoltage generationutilizing a liquid electrolyte for hole conduction. While such devicesare attractive in terms of their inherent simplicity and use ofenvironmentally stable WBG material, the relatively low absorptioncoefficients of the photodyes necessitates use of extremely thick“mesoscopic” TiO₂ films for adequate capture of incident photons.Disadvantageously, however, this results in greater opportunity for lossof excited electron-hole pairs (“excitons”) due to recombination andthermalization. Consequently, the efficiency of solar conversion of suchdevices is generally limited to ˜10%.

According to a variation of this approach, quantum dots (“QD's”), i.e.,nano-dimensioned semiconductor particles that confine the motion ofconduction band electrons, valence band holes, or pairs of conductionband electrons and valence band holes (“excitons”) in all three spatialdirections, are substituted for the abovementioned dye molecules (see,e.g., U.S. Pat. Nos. 6,861,722 B2 and 7,042,029 B2). Contact betweenporous TiO₂ films and the QD's, may for example, be accomplished viaabsorption, utilizing a porous TiO₂ body and a colloidal solution of theQD, or produced via an in situ process. PV effects have been observedwith a number of semiconductor-based QD's, including InP, CdSe, CdS, andPbS. Advantages of QD's vis-à-vis dye molecules in solar PV applicationsinvolving sensitization of WBG's include better tunability of opticalproperties via size control of the QD particles and betterheterojunction formation with solid, rather than liquid hole conductors.

As explained below, another advantageous capability of QD-sensitized PVcells is the production of quantum yields >1 by impact ionization,sometimes referred to as the “inverse Auger effect”. Since the inverseAuger effect is not possible with dye-sensitized PV cells, much higherconversion efficiencies are possible with solid state heterojunctionQD-sensitized PV cells.

Referring to FIG. 1, shown therein, in schematic form, is a band diagramfor solid state heterojunction QD-sensitized PV cells, wherein: CB andVB respectively indicate the conduction band and valence band energiesof the QD and WBG materials (illustratively PbS and TiO₂, respectively),E_(F) represents the respective Fermi energy levels, and Φ_(A) indicatesthe respective electron affinities (work functions). In order for chargeinjection (sensitization) from the QD to the WBG material to occur, thedifference between the work function Φ_(A) and conduction band energy CBmust be greater for the WBG than for the QD.

Design rules for the materials selection process for fabricating solidstate heterojunction QD-sensitized PV cells such as shown in FIG. 1include:

1. the minimum size of the QD, i.e., D_(min)=(πh)/2m_(e)*ΔE_(c))^(1/2),where m_(e)* is the effective mass of the electron, and ΔE_(c) is thedifference between the energies of the CB's of the QD and WBG materials,wherein the bandgap energy E_(g) of the WBG semiconductor material is≧2.0 eV, preferably ≧3.0 eV; and the bandgap energy E_(g) of the QDsemiconductor material is <2.0 eV, preferably <1.0 eV; and

2. the exciton radius, a_(x), is larger than the QD radius, wherea_(x)=ε_(r)/M*a_(H), wherein is the relative permittivity andM*=(M_(e)*M_(h)*)/(M_(e)*+M_(h)*) and M_(e)* and M_(h)* denote theeffective masses of the electrons and holes in units of the electronrest mass and a_(H) is the Bohr radius of 5.29×10⁻¹¹ m. The QD radiusD_(min) ranges from about 3 to about 15 nm, and is typically about 5-9nm.

The term “exciton radius” a_(x) refers to the average physicalseparation between the electron and hole of the electron-hole pairwithin the semiconductor. If the size of the semiconductor particle,i.e., QD, is less than or equal to the exciton radius, quantumconfinement occurs and the energy levels in the QD are discrete andcontinuous energy level bands are no longer formed. Typical values ofa_(x) range between about 2 and about 20 nm.

Adverting to FIG. 2, schematically illustrated therein is a band diagramfor describing the increase in photon conversion efficiency afforded byQD's, wherein enhanced photocurrent is obtained when energetic (“hot”)charge carriers (e⁻h⁺ pairs or excitons) produce a second (or even athird, etc.) e⁻h⁺ pair or exciton via impact ionization, a process whichis the inverse of an Auger recombination process wherein two e⁻h⁺ pairsrecombine to form a single, highly energetic e⁻h⁺ pair. In order for theinverse Auger recombination process to result in increased photonconversion efficiency, the rate at which impact ionization occurs mustbe greater than the rate of carrier “cooling” and other relaxationprocesses for hot carriers.

Referring now to FIG. 3, schematically illustrated therein is a banddiagram illustrating the mechanism of operation of a solid stateheterojunction QD-sensitized PV cell comprising a layer of a WBGsemiconductor material (i.e., E_(g)≧2.0 eV, preferably ≧3.0 eV), e.g.,TiO₂, forming a heterojunction with a layer of a narrower bandgap QDsemiconductor material (i.e., E_(g)<2.0 eV, preferably <1.0 eV), e.g.,PbS. The latter is contacted at an opposite interface with a holeconductor, e.g., a layer of a hole conductive semiconductor material.Respective contacts (output terminals) are formed on the TiO₂ and holeconductive semiconductor layers for obtaining an electrical output fromthe cell for supply to a load device. As shown in the figure, absorptionof a photon in the QD results in formation of an energetic e⁻h⁺ pair (orexciton) in the QD, which e⁻h⁺ pair (or exciton) then produces a seconde⁻h⁺ pair in the QD via impact ionization. Both excited electrons in theconduction band of the QD may then be efficiently transferred to theconduction band of the electron conductive WBG semiconductor, and thenceto the negative output terminal. Hole transport occurs in the oppositedirection via the valence bands of the QD and hole conductivesemiconductor to the positive output terminal. FIGS. 4(A) and 4(B)schematically illustrate the directions of electron and hole travel,respectively, in the QD-sensitized PV cell of FIG. 3. As shown in FIG.4(A), if any portion of the QD is in interfacial contact with thenegative output terminal, excited electrons present in the QD may alsotravel directly to the negative output terminal to produce photocurrent.

Notwithstanding the potential for increased photon conversion efficiencyafforded by the solid state heterojunction QD-sensitized PV cells suchas described above, the heretofore utilized manufacturing techniquescomprising absorption of the QD material from solution onto a porousbody of a WBG semiconductor material, as well as in situ processing,result in relatively low surface-to-volume ratios of the WBGsemiconductor particles (e.g., TiO₂), which when combined with therelatively low coverage of the WBG particle surfaces with QD's, limitsthe actual performance and viability of such devices.

Accordingly, there exists a clear need for improved solid stateheterojunction QD-sensitized PV cells capable of performing in optimalmanner at high solar photon conversion efficiencies. Further, thereexists a clear need for improved methodology for fabrication of suchimproved solid state heterojunction QD-sensitized PV cells incost-effective manner utilizing readily available manufacturinginstrumentalities and technologies.

DISCLOSURE OF THE INVENTION

An advantage of the present invention is an improved quantum dot (QD)sensitized wide bandgap (WBG) semiconductor heterojunction photovoltaic(PV) device.

Another advantage of the present invention is an improved quantum dot(QD) sensitized wide bandgap (WBG) semiconductor heterojunctionphotovoltaic (PV) device with an active PV layer having a substantiallyincreased QD content.

Still another advantage of the present invention is an improved quantumdot (QD) sensitized wide bandgap (WBG) semiconductor heterojunctionphotovoltaic (PV) device with dual-sided irradiation capability.

Yet another advantage of the present invention is an improved method offabricating quantum dot (QD) sensitized wide bandgap (WBG) semiconductorheterojunction photovoltaic (PV) devices.

A further advantage of the present invention is an improved method offabricating quantum dot (QD) sensitized wide bandgap (WBG) semiconductorheterojunction photovoltaic (PV) devices with active PV layers havingsubstantially increased QD content.

A still further advantage of the present invention is an improved methodof fabricating quantum dot (QD) sensitized wide bandgap (WBG)semiconductor heterojunction photovoltaic (PV) devices with dual-sidedirradiation capability.

These and additional advantages and other features of the presentinvention will be set forth in the description which follows and in partwill become apparent to those having ordinary skill in the art uponexamination of the following or may be learned from the practice of thepresent invention. The advantages of the present invention may berealized as particularly pointed out in the appended claims.

According to an aspect of the present invention, the foregoing and otheradvantages are obtained in part by an improved quantum dot (QD)sensitized wide bandgap (WBG) semiconductor heterojunction photovoltaic(PV) device, comprising:

a wide bandgap (WBG) semiconductor material with E_(g)≧2.0 eV, in theform of a 2-dimensional matrix defining at least two open spaces; and

a narrower bandgap semiconductor material with E_(g)<2.0 eV, in the formof quantum dots (QD's) filling the open spaces defined by the matrix ofWBG semiconductor material and establishing a heterojunction therewith.

In accordance with embodiments of the present invention, the devicefurther comprises an electron conductive layer adjacent a first side ofthe active PV layer; a hole conductive layer adjacent a second side ofthe active PV layer; and an electrode layer adjacent the hole conductivelayer. Each QD is columnar- or grain-shaped and extends from theelectron conductive layer to the hole conductive layer; the WBG andnarrower bandgap semiconductor materials are immiscible; each QD isisolated by the matrix of WBG semiconductor material; the WBGsemiconductor material has a bandgap E_(g)>3.0 eV and is an electronconductive material selected from the group consisting of: TiO₂, ZnS,ZnO, Ta₂O₅, Nb₂O₅, and SnO₂; and the narrower bandgap semiconductormaterial has a bandgap E_(g)<1.0 eV and is selected from the groupconsisting of: PbS, InP, InAs, CdS, CdSe, CdTe, Bi₂S₃, and AlSb.

Another aspect of the present invention is an improved quantum dot (QD)sensitized wide bandgap (WBG) semiconductor heterojunction photovoltaic(PV) device, comprising:

-   -   (a) an electron conductive layer;    -   (b) an active photovoltaic (PV) layer adjacent the electron        conductive layer;    -   (c) a hole conductive layer adjacent the active PV layer; and    -   (d) an electrode layer adjacent the hole conductive layer;        wherein the active PV layer comprises:        -   (i) a wide bandgap (WBG) semiconductor material with            E_(g)≧2.0 eV, in the form of a 2-dimensional matrix defining            at least two open spaces; and        -   (ii) a narrower bandgap semiconductor material with            E_(g)<2.0 eV, in the form of quantum dots (QD's) filling the            open spaces defined by the matrix of WBG semiconductor            material and establishing a heterojunction therewith.

In accordance with embodiments of the present invention, the QD'sconstitute from about 40 to about 90 vol. % of the active PV layer,typically about 70 vol. % of the active PV layer; each QD is columnar-or grain-shaped and extends from the electron conductive layer to thehole conductive layer; the WBG and narrower bandgap semiconductormaterials are immiscible; and each QD is isolated by the matrix of WBGsemiconductor material.

According to the present invention, each columnar- or grain-shaped QDhas a physical dimension within the range from about 2 to about 10 nmfor exhibiting quantum containment effects; and the active PV layer isfrom about 2 to about 20 nm thick, preferably from about 2 to about 10nm thick.

Preferably, the WBG semiconductor material has a bandgap E_(g)≧3.0 eV;and the narrower bandgap semiconductor material has a bandgap E_(g)<1.0eV.

According to embodiments of the present invention, the WBG semiconductormaterial is an electron conductive material selected from the groupconsisting of: TiO₂, ZnS, ZnO, Ta₂O₅, Nb₂O₅, and SnO₂; and the narrowerbandgap semiconductor material is selected from the group consisting of:PbS, InP, InAs, CdS, CdSe, CdTe, Bi₂S₃, and AlSb. Preferably, the WBGsemiconductor material is anatase TiO₂; and the narrower bandgapsemiconductor material is PbS.

In accordance with embodiments of the present invention, the electronconductive layer is comprised of an n-type semiconductor material, andis preferably transparent and formed over a surface of a transparentsubstrate.

According to preferred embodiments of the present invention, the n-typesemiconductor material comprises a layer of a transparent conductiveoxide (TCO) material selected from the group consisting of: SnO₂:F, ZnO,SrRuO₃, In₂O₃—SnO₂ (ITO), and CdSnO₄, the layer comprising a granular,nano-textured surface in direct, preferably epitaxial, contact with theactive PV layer.

In accordance with embodiments of the present invention, adhesion andseed layers are present intermediate the surface of the transparentsubstrate and the TCO layer; the adhesion layer comprising at least onematerial selected from the group consisting of: Ti, Ta, CrTa, and CrTi;and the seed layer comprising at least one material selected from thegroup consisting of: Al, Au, Ag, Pt, Pd, Cu, Ni, Rh, Ru, Co, Re, and Ti.Preferably, the adhesion layer comprises Ti and the seed layer comprisesAu.

According to embodiments of the present invention, the hole conductivelayer comprises a semiconductor material (“SC”) wherein(Φ_(A)-VB)_(SC)>(Φ_(A)-VB)_(QD), e.g., Si, p-Si, GaAs, or p-GaAs; andthe electrode layer is comprised of an electrically conductive materialselected from the group consisting of: Hf, Au, Ni, Al, Cu, Pt, Pd, andTCO materials. Preferably, each of the electron conductive and electrodelayers is light transmissive, whereby the device is operable viairradiation of either or both light transmissive layers.

In accordance with embodiments of the present invention, the devicefurther comprises:

(e) electrical contacts to each of the electron conductive and electrodelayers.

Another aspect of the present invention is an improved method offabricating a quantum dot (QD) sensitized wide bandgap (WBG)semiconductor heterojunction photovoltaic (PV) device, comprising stepsof:

-   -   (a) providing an electron conductive layer;    -   (b) forming an active photovoltaic (PV) layer in contact with        the electron conductive layer;    -   (c) forming a hole conductive layer in contact with the active        PV layer; and    -   (d) forming an electrode layer in contact with the hole        conductive layer;    -   wherein step (b) comprises:        -   (i) providing a wide bandgap (WBG) semiconductor material            with E_(g)≧2.0 eV;        -   (ii) providing a narrower bandgap semiconductor material            with E_(g)<2.0 eV; and        -   (iii) forming a 2-dimensional matrix comprising the WBG            material, the matrix defining a plurality of open spaces;            and        -   (iv) filling the open spaces defined by said matrix of WBG            semiconductor material with quantum dots (QD's) of the            narrower bandgap semiconductor material, the narrower            bandgap and WBG semiconductor materials forming a            heterojunction therebetween.

In accordance with embodiments of the present invention, steps (b)(iii)and (b)(iv) occur substantially simultaneously and each comprise aphysical vapor deposition (PVD) process. Preferably, steps (b)(iii) and(b)(iv) are performed substantially simultaneously by co-sputterdeposition of the narrower bandgap and WBG semiconductor materials.

According to preferred embodiments of the present invention, steps(b)(iii) and (b)(iv) are performed according to process conditionstypical of Zone 1 of a Thornton diagram and include low to moderatesubstrate temperatures of about 200° C. and relatively high sputter gaspressures >20 mTorr of inert gas. Steps (b)(iii) and (b)(iv) may furthercomprise addition of at least one gas comprising at least oneconstituent element of the narrower bandgap and WBG semiconductormaterials.

In accordance with embodiments of the present invention, the QD'sconstitute from about 40 to about 90 vol. % of the active PV layerformed in step (b), typically about 70 vol. %.

Preferably, steps (b)(i) and (b)(ii) comprise providing immiscible WBGand narrower bandgap semiconductor materials; steps (b)(iii) and (b)(iv)form columnar- or grain-shaped QD's which extend from the electronconductive layer to the hole conductive layer, each QD being isolated bythe matrix of WBG semiconductor material; and each columnar- orgrain-shaped QD formed in steps (b)(iii) and (b)(iv) has a physicaldimension within the range from about 2 to about 10 nm for exhibitingquantum containment effects.

According to embodiments of the present invention, the active PV layerformed in steps (b)(iii) and (b)(iv) is from about 2 to about 20 nmthick, preferably from about 2 to about 10 nm thick; step (b)(i)preferably comprises providing a WBG semiconductor material having abandgap E_(g)≧3.0 eV; and step (b)(ii) preferably comprises providing anarrower bandgap semiconductor material having a bandgap E_(g)<1.0 eV.

Embodiments of the present invention include those wherein step (b)(i)comprises providing a WBG semiconductor material as an electronconductive material selected from the group consisting of: TiO₂, ZnS,ZnO, Ta₂O₅, Nb₂O₅, and SnO₂; and step (b)(ii) comprises providing anarrower bandgap semiconductor material selected from the groupconsisting of: PbS, InP, InAs, CdS, CdSe, CdTe, Bi₂S₃, and AlSb.Preferably, step (b)(i) comprises providing anatase TiO₂ as the WBGsemiconductor material; and step (b)(ii) comprises providing PbS as thenarrower bandgap semiconductor material.

In accordance with embodiments of the present invention, step (a)comprises providing an electron conductive layer comprised of an n-typesemiconductor material. Preferably, step (a) comprises providing atransparent n-type semiconductor layer formed over a surface of atransparent substrate. E.g., step (a) comprises providing the n-typesemiconductor material as a layer of a transparent conductive oxide(TCO) material selected from the group consisting of: SnO₂:F, ZnO,SrRuO₃, In₂O₃—SnO₂ (ITO), and CdSnO₄, wherein step (a) comprisesproviding the layer of TCO material with a granular, nano-texturedsurface; and step (b) comprises forming the active PV layer in directcontact with the layer of TCO material. Preferably, the interfacebetween the active PV layer and the layer of TCO material is epitaxial.

Preferably, step (a) comprises forming the layer of TCO material bymeans of a 2-stage sputter deposition process comprising a first stageutilizing a low pressure sputtering atmosphere from about 1 to about 10mTorr for deposition of an initial 2-90 nm thickness of the TCO layer,and a second stage utilizing a higher pressure sputtering atmospherefrom about 10 to about 200 mTorr for deposition of the final 1-10 nmthickness of the TCO layer.

According to embodiments of the present invention, step (a) furthercomprises providing adhesion and seed layers intermediate the surface ofthe transparent substrate and the layer of TCO material; the adhesionlayer comprises at least one material selected from the group consistingof: Ti, Ta, CrTa, and CrTi; and the seed layer comprises at least onematerial selected from the group consisting of: Al, Au, Ag, Pt, Pd, Cu,Ni, Rh, Ru, Co, Re, and Ti. Preferably, the adhesion layer comprises Tiand the seed layer comprises Au.

In accordance with embodiments of the present invention, step (c)comprises providing a hole conductive layer comprised of a semiconductormaterial (“SC”), wherein (Φ_(A)-VB)_(SC)>(Φ_(A)-VB)_(QD), e.g., Si,p-Si, GaAs, or p-GaAs; and step (d) comprises providing an electrodecomprised of an electrically conductive material selected from the groupconsisting of: Hf, Au, Ni, Al, Cu, Pt, Pd, and TCO materials.

Preferably, the electron conductive layer provided in step (a) and theelectrode layer formed in step (d) are each light transmissive, wherebythe device is operable via irradiation of either or both lighttransmissive layers.

According to embodiments of the present invention, the method furthercomprises:

(e) a step of forming electrical contacts to each of the electronconductive and electrode layers.

Additional advantages and aspects of the present invention will becomereadily apparent to those skilled in the art from the following detaileddescription, wherein embodiments of the present invention are shown anddescribed, simply by way of illustration of the best mode contemplatedfor practicing the present invention. As will be described, the presentinvention is capable of other and different embodiments, and its severaldetails are susceptible of modification in various obvious respects, allwithout departing from the spirit of the present invention. Accordingly,the drawings and description are to be regarded as illustrative innature, and not as limitative.

BRIEF DESCRIPTION OF THE DRAWINGS

The following detailed description of the embodiments of the presentinvention can best be understood when read in conjunction with thefollowing drawings, in which the various features are not necessarilydrawn to scale but rather are drawn as to best illustrate the pertinentfeatures, wherein:

FIG. 1 is a band diagram schematically illustrating a solid-stateheterojunction QD-sensitized PV cell;

FIG. 2 is a band diagram schematically illustrating exciton generationin QD's via impact ionization (inverse Auger effect);

FIG. 3 is a band diagram schematically illustrating exciton generationin a solid-state heterojunction QD-sensitized PV cell;

FIGS. 4(A) and 4(B) schematically illustrate the directions of electronand hole travel, respectively, in the QD-sensitized PV cell of FIG. 3;

FIG. 5 is a simplified, schematic cross-sectional view of anillustrative, but non-limitative, embodiment of a solid-stateheterojunction QD-sensitized PV cell according to the present invention;

FIG. 6 is a simplified, schematic plan view of 2-dimensionalhoneycomb-type structure of the active PV layer of QD-sensitized PVcells according to the present invention;

FIG. 7 is a Thornton-type diagram illustrating various crystallographicstructures/morphologies of films obtainable as a function of depositionparameters; and

FIGS. 8(A) and 8(B) schematically illustrate the impactionization/inverse Auger process and 2-sided irradiation capability ofQD-sensitized PV cells according to the present invention.

DESCRIPTION OF THE INVENTION

The present invention is based upon recognition that solid-stateheterojunction QD-sensitized PV cells can be fabricated withsubstantially improved viability when used as solar energy-to-electricalenergy conversion devices than heretofore possible via conventionalmethodologies, including absorption of the QD material from colloidalsolution and in situ processing. According to the present invention, theactive photovoltaic layer of solid-state heterojunction QD-sensitized PVcells is a novel granular film structure comprising a columnar-shaped,2-dimensional matrix including a substantially greater volume (henceamount) of QD material, relative to WBG material, than heretoforeafforded by the aforementioned conventional methodologies, such asabsorption of QD material from solution and in situ processing.

More specifically, according to the present invention, a substrate(preferably transparent) with an electrically conductive granular layerthereon (e.g., transparent conductive oxide, “TCO”, such as anindium-tin oxide, “ITO”, layer) having peaks and valleys and a desiredcrystallographic orientation is provided in an initial step, and in asubsequent step an active photovoltaic (“PV”) layer comprising QD andWBG material is co-deposited thereon, e.g., by a physical vapordeposition (PVD) process such as sputtering. The QD's nucleate asprecipitates filling the spaces in the 2-dimensional matrix of WBGmaterial, with the volume fraction of the QD material in the active PVlayer being in the range from about 40 to about 90 vol. %, typicallyabout 70 vol. %.

According to the invention, the QD material is co-deposited with theimmiscible WBG material, e.g., by sputter deposition utilizing separatetargets or a composite target comprised of both materials, such that theQD material nucleates as a precipitate at the peaks of the electricallyconductive ITO layer and forms columns (or grains) extendingsubstantially orthogonally therefrom to a hole conductive layer, whereasthe WBG material migrates to and segregates at the boundaries betweenadjacent columns (grains) of QD material. A film structure wherein QDmaterial fills the spaces defined by a 2-dimensional matrix of WBGsemiconductor material as a grain boundary segregant is thus formed,wherein the QD material occupies as much as 90 vol. % of the resultantfilm. The particles (columns) of QD material are well isolated, withdimensions in the nm range, i.e., consistent with the requirements forquantum confinement. The result is an active PV layer comprising aQD-sensitized WBG semiconductor material, including a sufficiently largearea of QD-WBG contact, thereby well-suited for obtaining devices withenhanced PV conversion efficiencies via impact ionization (inverse Augerrecombination), as described above.

Referring now to FIG. 5, shown therein is a simplified, schematiccross-sectional view of an illustrative, but non-limitative, embodimentof a solid-state heterojunction QD-sensitized PV cell 10 according tothe present invention. As shown, cell 10 comprises a stacked layerstructure including, in sequence: a (preferably transparent) substrate1, an adhesion/seed layer 2A/2B, an electron conductive layer 3, anactive PV layer 4, a hole conductive layer 5, and a (preferablytransparent) electrode 6. Electrical contacts 7 and 8 to the electronconductive and electrode layers 3 and 6, respectively, provideelectrical outputs for connection to a load device 9.

An illustrative, but non-limitative, sequence of process steps forfabricating cell 10 via PVD processing, e.g., co-sputtering, isdescribed in detail in the following:

In an initial step, a substrate 1 is provided, comprising a materialselected from the group consisting of glass, metals, alloys, polymers,ceramics, and composites and laminates thereof, with glass or othertransparent material being preferred for facilitating irradiation of theactive PV layer. A thin adhesion layer 2A, i.e., ˜2 nm thick, of atleast one material selected from the group consisting of Ti, Ta, CrTa,and CrTi is formed on the surface of the substrate 1, it generally beingadvantageous to form the adhesion layer 2A under deposition conditionsyielding a smooth finished surface, e.g., a regime utilizing a lowpressure sputtering atmosphere, such as from about 1 to about 5 mTorrAr, Kr, or Xe. A seed layer 2B from about 1 to about 5 nm thick andcomprising at least one material selected from the group consisting ofAl, Au, Ag, Pt, Pd, Cu, Ni, Rh, Ru, Co, Re, and Ti is then formed undersimilar conditions in overlying contact with the adhesion layer 2A. Theseed layer 2B functions to provide a desired crystallographic textureand adequate lattice match to subsequently deposited overlying layers.Suitable alternative materials for the seed layer 2B include alloycompositions primarily comprising fcc or hcp metals with additions ofother fcc or hcp metals or even bcc materials. A preferred adhesionlayer/seed layer 2A/2B combination is Ti/Au.

In the next step according to the inventive methodology, a firstelectrode 3 comprised of a transparent electrically conductive material,e.g., a transparent conductive oxide (“TCO”) such as SnO₂:F, ZnO,SrRuO₃, In₂O₃—SnO₂ (“ITO”), and CdSnO₄, is formed on the adhesionlayer/seed layer 2A/2B in a 2-stage sputtering process. A low pressuresputtering atmosphere, e.g., from about 1 to about 10 mTorr, is utilizedin the first stage for deposition of the initial 2-90 nm thickness ofthe TCO, and a higher pressure sputtering atmosphere, e.g., from about10 to about 200 mTorr, is utilized in the second stage for deposition ofthe final 1-10 nm thickness of the TCO. The 2-stage sputtering processcreates a granular, nano-textured TCO surface necessary for achieving2-12 nm separation of the subsequently deposited QD columns or grains inthe overlying active PV layer.

According to the next step of the inventive methodology, the active PVlayer 4 comprised of a WBG semiconductor material (i.e., ≧2.0 eV,preferably ≧3.0 eV), and a narrower bandgap QD semiconductor material(i.e., <2.0 eV, preferably <1.0 eV) is deposited on the granular,nano-textured surface of the TCO layer 3 via a PVD process, preferably aco-sputter deposition process utilizing separate targets or targetscomprised of mixtures or segments of each material, and DC, AC, or RFplasma generation. Depending upon the electrical characteristics of theparticular targets employed for the co-deposition process, DC- orRF-magnetron or DC- or RF-diode cathodes may be utilized. Suitablematerials for use as the WBG semiconductor include electron conductivematerials such as TiO₂, ZnS, ZnO, Ta₂O₅, Nb₂O₅, and SnO₂, with anataseTiO₂ being preferred; and suitable materials for use as the QD materialinclude PbS, InP, InAs, CdS, CdSe, CdTe, Bi₂S₃, and AlSb, with PbS beingpreferred. The QD material is co-deposited with the immiscible WBGmaterial, such that the QD material nucleates at the peaks of the TCOlayer 3 and forms columns (or grains) extending substantiallyorthogonally therefrom, whereas the WBG material migrates to andsegregates at the boundaries between adjacent columns (grains) of QDmaterial to form a 2-dimensional matrix, as for example, thesubstantially honeycomb-shaped matrix schematically shown in plan viewin FIG. 6, in which the QD material fills the spaces of the2-dimensional matrix formed by the WBG semiconductor material andestablishes a heterojunction therewith.

The sputter deposition parameters utilized for the co-deposition processare similar to the deposition parameters utilized for the seconddeposition phase of the TCO layer 3 (described supra), and are similarto those utilized for forming the sharply separated columnar grains inZone 1 of a Thornton diagram (see, e.g., J. Vac. Sci. Technol. 11,666-670 (1974) or U.S. Pat. No. 4,557,981). In contrast with QD-WBGactive layers produced by conventional methodologies, an active PV layer4 comprising a 2-dimensional matrix of QD material with WBG material asa grain boundary segregant is formed according to the invention, whereinthe QD material constitutes a substantially increased portion of thevolume of the active PV layer 4, i.e., from about 40 to about 90 vol. %of the resultant film, typically about 70 vol. %.

Process conditions typical of Zone 1 of the Thornton diagram include lowto moderate substrate temperature, e.g., <40% homologous temperature(defined as T_(substrate)/T_(melting)), preferably about 200° C., andrelatively high sputter gas pressures >20 mTorr of inert gas (e.g., Ar),preferably >30 mTorr. This combination of process parameters yields thinfilms with columnar (or granular) grain structures with varying amountsof porosity between neighboring grains, and is ideal for facilitatingsegregation of immiscible materials, since the columnar grain structuresform the QD's while the immiscible WBG material is accumulated/trappedat the porous grain boundaries. The resultant structure may resemble ahoneycomb when viewed in plan view (as in FIG. 6), where the WBGsemiconductor material forms a 2-dimensional, substantiallyhoneycomb-shaped matrix or lattice and the QD semiconductor material(shaded in the figure) occupies the spaces defined by the matrix orlattice.

The particles (columns) of QD material are well isolated, withdimensions in the nm range, consistent with the requirements for quantumconfinement. Also in contrast with conventional QD-WBG active layermaterials and QD-WBG PV cells, the columnar-shaped grains extend forsubstantially the entire thickness of the active PV layer 4, and contactthe electron conductive layer 3 and the subsequently formed holeconductive layer 5.

Proper adjustment of the constituent stoichiometries of the WBG matrix(segregant) material, e.g., Ti and O₂ of TiO₂, and QD columnar material,e.g., Pb and S of PbS, may require addition of gases such as H₂S and/orO₂ during the co-deposition process. Anatase TiO₂ is a preferable matrixmaterial (segregant) in view of its ability to accept charge injectionfrom the neighboring QD material at the grain boundaries. In addition,it is immiscible in the material of the QD, e.g., PbS, and thereforedrifts along the growing surfaces of the QD columns or grains untilcaptured in the physically recessed regions of the grain boundaries.Simultaneously, the QD semiconductor material forms an epitaxialinterface with the TCO layer 3, e.g., ITO. The co-deposition processtherefore leads to isolated QD semiconductor columns or grains withphysical dimension consistent with obtaining/exhibiting quantumconfinement effects, i.e., from about 2 to about 10 nm. A log-normaldistribution of grain diameters, d, occurs, and with optimization ofco-deposition parameters σ_(d)/d is less than ˜20%. Suitable thicknessesof the QD-WBG active PV layer according to the invention range fromabout 2 to about 20 nm, with from about 2 to about 10 nm preferred.

In the next step according to the inventive methodology, a holeconductive semiconductor layer 5 is deposited on the active PV layer 4via a sputtering process performed under conditions similar to thoseutilized for deposition of the seed layer 2B. In this regard, inorganicsemiconductor (SC) materials wherein (Φ_(A)-VB)_(SC)>(Φ_(A)-VB)_(QD),e.g., Si, p-Si, GaAs, or p-GaAs, are preferred, inasmuch as they arecapable of accepting holes from the QD (absorber) as well as injectingelectrons from the contact back into the QD. The hole conductor layer 5is from about 1 to about 20 nm thick and is optimized for maximum holeconductivity and minimum electron tunneling current. The thickness ofthe hole conductor layer is desirably minimized in order to achievetransparency, thereby facilitating exposure of either side of the deviceto light and use as an electricity generating window covering.

An electrode layer 6 is then formed over the hole conductor layer, andmay comprise a relatively thin metal layer, i.e., from about 2 to about20 nm thick and comprised of, e.g., Hf, Au, Ni, Al, Cu, Pt, or Pd, or arelatively thick TCO layer, i.e., from about 10 to about 100 nm thick,and comprised of ITO. Process conditions for forming the secondelectrode layer 6 are not critical, and the latter may, if desired ornecessary for protection from deleterious environmental effects, becovered with a relatively thick, impervious layer of a transparent glassor polymeric material.

Finally, respective electrical output contacts 7 and 8 are affixed tothe electron conductive layer 3 and electrode layer 6 in conventionalmanner in order to obtain electrical output from the cell for supply toa load 9.

An advantage of the present invention is the ability to fabricate cellsfrom optically transparent substrate, adhesion/seed layer, electronconductor, and electrode materials, whereby irradiation of either orboth sides is possible. Referring to FIGS. 8(A) and 8(B), schematicallyillustrated therein are the impact ionization/inverse Augerrecombination process and 2-sided irradiation capability of solid-stateheterojunction QD-sensitized PV cells according to the presentinvention.

The present invention therefore advantageously provides improved, highphoton conversion efficiency, solid-state heterojunction, quantumdot-sensitized wide bandgap (QD-WBG) photovoltaic (PV) devices whichavoid the above-described limitations imposed on the quantum (photonconversion) efficiency of prior QD-WBG devices fabricated according toconventional methodologies. The QD-WBG PV devices of the presentinvention are especially useful when utilized as solar cells for thedirect conversion of solar radiation to electricity. Further, QD-WBG PVdevices according to the present invention can be fabricated incost-effective manner utilizing well-known PVD techniques such assputter deposition.

In the previous description, numerous specific details are set forth,such as specific materials, structures, processes, etc., in order toprovide a better understanding of the present invention. However, thepresent invention can be practiced without resorting to the detailsspecifically set forth. In other instances, well-known processingmaterials and techniques have not been described in detail in order notto unnecessarily obscure the present invention.

Only the preferred embodiments of the present invention and but a fewexamples of its versatility are shown and described in the presentdisclosure. It is to be understood that the present invention is capableof use in various other combinations and environments and is susceptibleof changes and/or modifications within the scope of the inventiveconcept as expressed herein.

1. A quantum dot (QD) sensitized wide bandgap (WBG) semiconductorheterojunction photovoltaic (PV) device, including an activephotovoltaic (PV) layer comprising: a wide bandgap (WBG) semiconductormaterial with Eg≧2.0 eV, in the form of a matrix defining at least twoopen spaces; and a narrower bandgap semiconductor material with Eg<2.0eV, in the form of quantum dots (QDs) filling said open spaces definedby said matrix and establishing a heterojunction therewith wherein eachQD of said QDs is isolated by said WBG semiconductor material andextends from an electron conductive layer to a hole conductive layer. 2.The device of claim 1, further comprising: an electron conductive layeradjacent a first side of said active PV layer; a hole conductive layeradjacent a second side of said active PV layer; and an electrode layeradjacent said hole conductive layer.
 3. The device of claim 2, wherein:each QD is columnar; said WBG and narrower bandgap semiconductormaterials are immiscible; and each QD is isolated by said matrix of WBGsemiconductor material.
 4. The device of claim 1, wherein: said WBGsemiconductor material has a bandgap Eg≧3.0 eV and is an electronconductive material selected from the group consisting of: TiO₂, ZnS,ZnO, Ta₂O₅, Nb₂O₅, and SnO₂; and said narrower bandgap semiconductormaterial has a bandgap Eg<1.0 eV and is selected from the groupconsisting of: PbS, InP, InAs, CdS, CdSe, CdTe, Bi₂S₃, and AlSb.
 5. Aquantum dot (QD) sensitized wide bandgap (WBG) semiconductorheterojunction photovoltaic (PV) device, comprising: (a) an electronconductive layer; (b) an active photovoltaic (PV) layer in contact withsaid electron conductive layer; (c) a hole conductive layer in contactwith said active PV layer; and (d) an electrode layer in contact withsaid hole conductive layer; wherein said active PV layer comprises: (i)a wide bandgap (WBG) semiconductor material with Eg≧2.0 eV, in the formof a matrix defining a plurality of open spaces; and (ii) a narrowerbandgap semiconductor material with Eg<2.0 eV, in the form of quantumdots (QDs) filling said open spaces defined by said matrix andestablishing a heterojunction therewith wherein each QD of said QDs isisolated by said WBG semiconductor material and extends from saidelectron conductive layer to said hole conductive layer.
 6. The deviceof claim 5, wherein: said QDs constitute from about 40 to about 90 vol.% of said active PV layer.
 7. The device of claim 6, wherein: said QDsconstitute about 70 vol. % of said active PV layer.
 8. The device ofclaim 5, wherein: (i) each QD is columnar; (ii) said WBG and narrowerbandgap semiconductor materials are immiscible; and (iii) each QD isisolated by said matrix.
 9. The device of claim 8, wherein: each saidcolumnar QD has a physical dimension within a range of about 2 to about10 nm for exhibiting quantum containment effects.
 10. The device ofclaim 8, wherein: said active PV layer is from about 2 to about 20 nmthick.
 11. The device of claim 10, wherein: said active PV layer is fromabout 2 to about 10 nm thick.
 12. The device of claim 5, wherein: (i)said WBG semiconductor material has a bandgap Eg≧3.0 eV; and (ii) saidnarrower bandgap semiconductor material has a bandgap Eg<1.0 eV.
 13. Thedevice of claim 5, wherein: (i) said WBG semiconductor material is anelectron conductive material selected from the group consisting of:TiO₂, ZnS, ZnO, Ta₂O₅, Nb₂O₅, and SnO₂; and (ii) said narrower bandgapsemiconductor material is selected from the group consisting of: PbS,InP, InAs, CdS, CdSe, CdTe, Bi₂S₃, and AlSb.
 14. The device of claim 13,wherein: (i) said WBG semiconductor material is anatase TiO₂; and (ii)said narrower bandgap semiconductor material is PbS.
 15. The device ofclaim 5, wherein: said electron conductive layer is comprised of a layerof an n-type semiconductor material.
 16. The device of claim 15,wherein: said n-type semiconductor layer is transparent and is formedover a surface of a transparent substrate.
 17. The device of claim 16,wherein: said n-type semiconductor layer comprises a layer of atransparent conductive oxide (TCO) material selected from the groupconsisting of: SnO₂:F, ZnO, SrRuO₃,In₂O₃—SnO₂ (ITO), and CdSnO₄.
 18. Thedevice of claim 17, wherein: said TCO layer comprises a granular,nano-textured surface in direct contact with said active PV layer. 19.The device of claim 18, wherein: an interface between said TCO layer andsaid active PV layer is epitaxial.
 20. The device of claim 18, furthercomprising: adhesion and seed layers intermediate said surface of saidtransparent substrate and said TCO layer.
 21. The device of claim 20,wherein: (i) said adhesion layer comprises at least one materialselected from the group consisting of: Ti, Ta, CrTa, and CrTi; and (ii)said seed layer comprises at least one material selected from the groupconsisting of: Al, Au, Ag, Pt, Pd, Cu, Ni, Rh, Ru, Co, Re, and Ti. 22.The device of claim 21, wherein: (i) said adhesion layer comprises Ti;and (ii) said seed layer comprises Au.
 23. The device of claim 5,wherein: said hole conductive layer comprises a semiconductor material.24. The device of claim 23, wherein: said hole conductive layercomprises one of Si, p-Si, GaAs, and p-GaAs.
 25. The device of claim 5,wherein: said electrode layer is comprised of an electrically conductivematerial selected from the group consisting of: Hf, Au, Ni, Al, Cu, Pt,Pd, and TCO materials.
 26. The device of claim 5, wherein: each of saidelectron conductive and electrode layers is light transmissive, wherebysaid device is operable via irradiation of one of either and both saidlight transmissive layers.
 27. The device of claim 5, furthercomprising: (e) electrical contacts to each of said electron conductiveand electrode layers.